The prerequisite of simulating the above two ions in water is an accurate description of bulk water from AIMD.
In the last few decades, AIMD simulations based on the density functional theory (DFT) [1]}, [2]}
have been widely utilized to characterize bulk water because the accuracy and efficiency are reasonably balanced. [3]}, [4]}, [5]}, [6]}, [7]}, [8]}
However, modelling of bulk water is non-trivial because several competing physical effects are present.
Specifically, bulk water at ambient conditions is comprised of water molecules embedded within
a continuously fluctuating H-bond network,
which origins from a delicate balance among various physical interactions including
covalent bonds, H-bonds, and van der Waals interactions. [9]}, [10]}, [11]}
Importantly, the balance between intra-molecular and inter-molecular interactions in liquid water largely depends on the choice of exchange-correlation (XC) functionals utilized in DFT.
Early AIMD simulations of water [12]}, [13]}
based on the LDA or GGA XC functionals predicted an over-structured H-bond network of water,
resulting in sluggish and rigid water with a low density. [14]}, [9]}
Later, adding van der Waals interactions upon GGA functionals has become a common practice to yield a better description of structural and dynamic properties of water, as well as restoring the water density at ambient conditions.
[9]}, [17]}, [18]}, [19]}, [10]}, [11]}
On one hand, the van der Waals interactions increase
the non-directional attraction forces between water molecules, which compensate for the underestimated water density. [10]}
On the other hand, the PBE0 hybrid functional [23]}, [24]} mitigates the
self-interaction error and better accounts for the molecular polarizability in liquid water simulations, [11]}
resulting in weakened HBs between water molecules that are closer to experiments.
The above two corrections draw the accuracy of simulating water towards experimental data.
